Speed sensors that rely on reflection of waves from objects may rely upon pattern-recognition techniques to determine the distance that a reflecting object and the speed sensor are displaced with respect to one another in a given time.
A specific example from the marine industry is a sonar speed sensor that is designed with two transmitter-receivers. Sonar signals can be transmitted from acoustic transmitter-receivers, reflected from objects or particles in the water, detected by each transmitter-receiver at different times, and then analyzed to look for changes in the reflection pattern and thus determine a relative displacement of a marine vessel with respect to the water with time. In particular, the pattern analysis has been done using cross-correlation and difference methods to compare data sets from the two transmitter-receivers.